I was randomly searching around to gather some more information on pinball machines and stumbled upon a really neat article written in 2005.
“What differentiates this project from the typical emulator is that no reference is made to the original programs encoded on the MPU firmware. Instead, I employed a black box, or clean room, method based on studying their function rather than their internal structure. For me, it made sense to interpret these 66 electrical connections in terms of their purpose in a closed-loop process control model. That is, each is either input, output, part of a feedback circuit or part of the power supply. The four main divisions of the pinball machine control system are the solenoids, switch matrix, feature lamps and digital displays. I intentionally left out the digital displays for the first prototype, which is why the apparatus uses the computer monitor to show the scores”
It’s really interesting to see how he managed to interface with the existing MPU with a PC and custom IO card. He also released the code he used — PMREK. It hasn’t been updated since 2005 but I imagine it can be pretty valuable.